Frequency modulation



Nov. 25, `194:1'. E. AEN 2,431,569

FREQUENCY y MODULATION l Filed 0Cb. 14, 1942 .slsf/frm cosa' 't' I HG.3. c'

ATTRNEY Patented Nov. 25, 1947 UNITED STATES PATENT OFFICE FREQUENCYMODULATION Application October 14, 1942, Serial No. 462,013

2 Claims. 1

The present invention relates to radio trans'- mission of the type inwhich the transmitted wave is frequency modulated. More particularly,the invention is concerned with the production of the frequencymodulated Wave by a method of direct phase modulation. Y

In my prior application Serial No. 450,596 of July 11, 1942, on a Methodof producing frequency modulated waves for radio transmission, I haveset forth the chief difficulties encountered in frequency modulatedtransmission, and have shown that the frequency modulated wave may beexpressed as u=U sin (Slet-Hb) where il/ is a term representing theintegrated intelligence and is given by2vrlcj`s(t)dt=21rkAfa(t)dt=j`a(t) dt, s(t)=Aa(t) being the intelligenceto be introduced into the wave. I have further recalled in the aforesaidprior application that the wave u=U sin (uct-ew) may be obtained bymixing a sinusoidal H. F. oscillation w=Ua sin Slet, of the centralfrequency 9e with an oscillating L. F. tension 'U=V cos 1,0, where thevalue of the angle is kfsw) di, on the assumption that it were possibleto produce, from the integrated intelligence s(t) such an oscillatingtension v=V cos ip. In said prior application, a practical solution wasfound by a particular utilization of the Kerr cell.

In the prior application Serial No. 453,890, led August 6, 1942, in thenames of Edouard Labin and Manuel Julio Kobilsky, a thermionic tube isdescribed whereby inter alia an output may be obtained which is asinusoidal function of an electrical quantity impressed on controlmembers provided in operative relationship with respect to an electronicbeam. It likewise describes general methods for determining the shapeand/or cornl position of secondary emission surfaces on the basis ofwhich surfaces capable of yielding a sinusoidal output may beconstructed. It, is therefore deemed unnecessary to detail in thepresent specification the construction of the forming device or themanner of designing the forming elements thereof.

Ii; will be sufficient therefore, for the purposes of the presentinvention to recall that the tubes of said prior specification ofEdouard Labin and Manuel Julio Kobilsky are of the kind comprising thegeneration of a beam of electrons which is passed through aspatialdisplacement zone on which is impressed an electrical quantity acting tocontrol the extent of the spatial displacement, the beam after leavingsaid spatial displacement zone passing to a forming zone the geometryand composition of which is such that the intensity of theelectriccurrent emerging therefrom is a (Cl. 179-4715)l 2 sinusoidalfunction of the spatial displacements of the said beam.

For the purposes of the present invention and in accordance with thehereinabove recalled theory, I use for the electrical quantitycontrolling the spatial displacement a varying and oscillatingelectrical quantity so that the output of the forming zone is asinusoidal function of such varying and oscillating electrical quantity.To obtain an output which shall be a sinusoidal function of the sum oftwo angles, as in frequency modulated transmission, two such formingzones are used, one in quadrature with the other to obtain the terms insin gb and cos 1,1/ and the two outputs are combined severally withsinusoidal functions of a central frequency obtained by impressing acentral frequency generated in a stabilized pilot, on respective phaseshifting zones in quadrature, the two combinations then being appliedsimultaneously to a common point of an electrical system, such as atransmitter.

For frequency modulated transmission, the varying and oscillatingelectrical quantity is the integrated intelligence so that the resultingwave is Ilt=U sin (9CH-if).

The fundamental advantage of employing the tube mentioned is the newpossibility of obtaining in one step values of ,b as high as a number oftimes 21|. l

It is, therefore, a principal object of the present invention toprovide, particularly in connection with frequency modulated radiotransmission, a new and improved method of generating an elecf tricalquantity varying as a sinusoidal function of the sum of two angles, eachof said angles being respectively proportional to one of two otherelectrical quantities.

A further object of the present invention is to provide a method forgenerating, for an electrical system, an electrical quantity varying asa sinusoidal function of the sum of two angles, which comprises thesteps of generating a stabilized central frequency, passing the centralfrequency simultaneously through two separate zones of phasedisplacement in quadrature with each other whereby to obtain twosinusoidal oscillations of central frequency in quadrature with eachother and thereby to provide factors which are sinusoidal functions inquadrature of one of said angles, generating a varying and oscillatingother electrical quantity, generating two electron beams, each beamcorresponding to one of said phase displacement zones, passing each beamthrough a respective spatial displacement zone and a forming zone thegeometry and composition of which is such as to give at its output anemerging electrical quantity which is a sinusoidal func- A furtherobject of the present invention is to provide a method for generating,for an' electrical system, an electrical quantity varying as asinusoidal function of the sum of two angles, which comprises the stepsof generating a stabilized central frequency, passing the centralfrequency simultaneously through two separate zones of phasedisplacement in quadrature with each other whereby to obtain twosinusoidal oscillations of central frequency in quadrature with eachother and thereby to provide factors which are sinusoidal functions inquadrature of one of said angles. generating a varying and oscillatingother electrical quantity, generating two electron beams in respectivegenerating zones while impressing on one generating Zone one of saidsinusoidal oscillations of central frequency and the other sinusoidaloscillation of central frequency on the other generating zone to impresson the respective beams sinusoidal variations in intensity in quadraturewith each other and representative of said one angle, passing eachintensity modulated beam through a respective spatial displacement Zonefollowed by a forming zone, the geometry and composition of which issuch as to impress on the output a modulation which is a sinusoidalfunction of the spatial displacement impressed on the respective beamduring its passage through the spatial displacement zone, controllingthe operation of each spatial displacement zone by impressing thereon,as a control factor, said varying and oscillating other electricalquantity, whereby to combine with the intensity modulation correspondingto the sinusoidal function of said one angle a modulation correspondingto a quadratured sinusoidal function of said other angle, and applyingthe two outputs of the forming Zones simultaneously to a common point ofsaid electrical system.

These and other objects and advantages of the present invention willbecome apparent in the course of the following detailed description inwhich reference is made to the accompanying drawings.

In the drawings:

Fig. 1 is a diagram illustrating the application of the invention to thegenerating of a frequency modulated wave.

Fig. 2 is a diagram illustrating a modied form of application.

Fig. 3 illustrates one form of apparatus suitable for the purposes ofthe invention.

Given the principle of obtaining the sinusoidal factors of the angle 1pby impressing on a beam of electrons spatial displacements in responseto a varying and oscillating electrical quantity, causing said beam toenter a forming zone and deriving from the forming zone a sinusoidalfunction of said varying oscillating electrical quantity, the

desired final result, namely an electrical quantity varying as thesinusoidal function of the sum of two angles one of which corresponds tosaid vary- -ing and oscillating electrical quantity, and the other ofwhich corresponds to a central frequency, may readily be obtained byconnecting the operative parts in a mannerwhich is in itself quiteclassical. Such assembly of the operative parts or inter-relationing ofthe several zones is illustrated diagrammatically in Fig, 1.

The elements PS1 and PS2, labelled Phase- Shifters, are adapted to giveto the oscillation ce originating from the stabilized pilot SP, twoforms inquadrature,

l1L1=Uo Sill 9ct, and u2=Uo COS Slet sinip, and 122=V0 cos rb The saidphase modulators are constituted in accordance with the teaching of thesaid prior patent application of E. Labin and M. J. Kobilsky. A suitablephase modulator is shown in Fig. 3 and comprises an electron dischargetube l0 having an evacuated and hermetically sealed envelope includinganindirectly heated cathode I2, a 'heating filament lil, an intensitycontrol grid I6 and a beamV concentrating means I8, so arranged thatfrom said beam concentrating means I8 there issues a beam of electronshaving a desired cross section. The tubeV also `comprisesbeamfdeflecting electrodes 253-20 arranged on the side of the beamconcentrator 20 remote from the cathode.

On the side of the deflecting electrodes20-20 remote from the cathodeand'in the path of the electron beam, there is provided a responseforming element 22 having a secondary electron emissive surface withsinusoidal undulations. A passive collector electrode 2li is providedfor collecting the secondary electrons provided by the element 22. Asuitable load impedance 25 is provided in the output circuit of thecollector electrode 24.

In operation the phase modulator of Fig. 3 provides anoutput voltage econtaining the desired argument. V cos gl/ or V sin 1p by reason of themovement of the electron beam under the influence of the Vdeilectingelectrodes 2li-2D whereby a voltage sd) applied to these electrodescauses the beam to scan the undulated surface of the element 22 andproduce a secondary emissive current to the collector 24 proportional tothe angle of incidence of the beam with respect to the surface of theelement. Whether the argument cos ip or sin 1,1/ is generated isdetermined by the initial or resting position of the beam with respectto the undulated surface of element 22 and one or the other function isobtained by suitable selection of the resting position of the beam withrespect to the position of the element.

The arrangement shown in Fig. 1 comprises two tubes of the foregoingtype such tubes constituting the phase-modulators PM1 and PM2 and byapplying the intelligence voltage s(t) to the defiecting electrodesZB-Zll thereof, the arguments cos 1p and sin 5b are produced.

The multiplication of each L. F. oscillation, ci, 112, with thecorresponding H. F. oscillation, u1, u2, is carried out in two classicalmodulators M1, M2, which have been designated intensity modulators todistinguish them from the phase modulators. To add the two products thusJob'- tained, mural-:U sin dat. cos d, and muzvz-:U .cos 9ct. sin yb, itis sufficient to apply the outputs of the intensity modulatorssimultaneously to a common point of an electrical system, such as atransmitter, as indicated at T in the figure.

An ordinary intensity modulation generates not only the products U sinmet. cos ,b, and U cos Slet. sin tb, but, at the plate of the modulatingtube, there also appear currents which reproduce the two individualoscillations applied. The L. F. oscillation will not give an outputvoltage, because the plate circuit will loe closed through .a ifiltercapable of .rejecting it completely. iB ut sucha filter cannot rejectthe.current opt, :located right in the middle Yof the .band which it musttransmit. However, a differential modulator of the classical type can beused for the intensity modulators to eliminate the central frequency bya process of internal opposition. Should the management of such amodulator prove to be too delicate a matter, due to a working frequencyne which is already high, it will be suflicient to use a method offrequency conversion and create the two modulations, in phase and inintensity, on a lower frequency ile. Furthermore, it should be`mentioned that the presence of a certain portion of pure centralfrequency beyond the modulating system, and hence up to within theaerial, does not do much damage to the reception of the useful wave, solong as such portion does not eX- ceed a value of some tenths (in power)of the useful wave. In fact, it is known that the reception of frequencymodulated waves olers a very good protection against interferencesignals the amplitude of which is a few times lower than that of thedesired signal, wherever the frequency of the former is located withrespect to that of the latter. The same may be said of any lack ofsymmetry between the two paths, which is translated by the persistenceof a lateral band at the output, alongside the desired wave. It is thusseen that the arrangement of the system of mixture is not critical.

The multiplication of an L. F. tension v1=V cos if by an H. F. voltageu1=Uo sin Q et, may, as shown in Fig. 2, be effected without thedifferential modulators of Fig, l, whenever the intensity V of the L. F.voltage can be controlled independently, In the arrangement shown inFig. 2 the outputs of the phase shifters PS1' and PS2 are each appliedto the intensity control .grid I6 of the tubes Ill constituting thephase modulators PM1 and PM2 to thereby intensity modulate therespective electron beams. 'Ihe intelligence voltage s t) is applied tothe dellecting electrodes 2li-20 of each of the tubes. In thisarrangement the desired multiplication is effected in each tube wherebythe output voltage e of the phase modulator PMi comprises the desiredargument sin Slet. cos tb and the output voltage of phase modulator PM2'comprises the argument cos Slet. sin 1p as indicated in Fig, 2. Let itbe supposed tha-t the phase modulator PM1 which provides V cos tlf froms(t) (or from \//=kfs(t) dt) operates without perturbations whatever maybe the amplitude V, and, more particularly, even if the amplitude Vvaries in time with an H. F. rhythm. Then it is purposely controlled bythe phase-shifted piloted frequency from PS1', that is to say, thecontrol voltage is made to be of the type V=V0 sin Slet, so that fromthe phase modulator PM1 the output is directly the product V0 sin 9ct.cos tb. The complementary phase modulation path PM2', which generatessin 1p,

would be modulated in intensity bythe phaseshifted piloted .oscillationin vquadrature cos Slet from PS2'. The combination, at a common point Tof the Aoutputs .of the two paths, effects the addition required .togive the complete wave. -In this embodiment, two differential modulatorsas well ias two .separate outputs for the two phase modulation paths aresuppressed, and the necessary devices are added to control the amplitude`of Ithe L. oscillations in accordance with :the rhythm of ythe F.oscillation. The net result of this is always -a simplification of thesystem.

It .can vbe Shown vthat in .order to obtain the ydesired resultant whichshall be a sinusoidal :function of the sum of the .two angles Slet andd, -it' will suice to lcombine the two output points :ofl the for-mingzones, This means in practice that a twin forming device with acollecting anode common to the two parts, may be used. The undesiredparts of the output which reproduce the piloted oscilaltions can beeliminated by opposing two voltages of appropriate amplitude takendirectly from the phase shifters, since within the range of frequenciesutilized, the accurate opposition of two voltages at a point remote fromthat of formation offers no special diculties. Such elimination isindicated in the broken lines in Fig. 2 leading from the phase shiftersPS1' and PS2', to an opposition zone ZO.

Calculation based on suitable assumed values demonstrates that theparasitic phase displacement introduced by differences in the length ofpath of the electron beam as it moves over a configured secondaryemission surface will not prove troublesome.

The only disadvantage of the intensity modulation carried out directlyin the tube, is that the amplitude of the useful electrical currentscannot be more than a fraction of the mean c'onstant part thereof, owingto the necessity for the linear control of the grid by the pilotedoscillation. Hence, in order to obtain for the proper intensity for saidamplitude a total emission a few times greater must be taken from thecathode of the tube. But this is still not a serious matter, if theconcentration system is carefully adjusted.

I claim:

1. A method of generating an electrical quantity varying as a sinusoidalfunction of the sum of two angles, comprising the steps of deriving twooscillations in phase quadrature from a wave having a stable frequency,generating a varying and oscillating electrical quantity, generating twoelectron beams, modulating the intensity of one of said beamsproportional to the amplitude of one of said oscillations, modulatingthe intensity of the other beam proportional to the amplitude of theother of said oscillations, deilecting each of said beams proportionalto the amplitude of the varying and oscillating electrical quantity,deriving from the intensity modulated and deflected electron beamssecondary emissive currents proportional to the amplitude of saidoscillations and sinusoidal functions in quadrature of said varying andoscillating electrical quantity, and combining the said secondaryemissive currents.

2. A method of generating an electrical quantity varying as a sinusoidalfunction of the sum of two angles, comprising the steps of deriving froma wave having a stable frequency two oscillations in phase quadrature,the argument of said wave constituting the first of said angles, whilethe second angle is proportional to a generated Vvarying and oscillatingelectrical quantity. generating two electron beams, modulating theintensity of one of said beams proportional to the amplitude of oneV ofsaid oscillations, modulating the intensity of the other beamproportional to the amplitude of the other of said oscillations,deflecting each of said beams proportional to the amplitude of thevarying and oscillating electrical quantity, deriving from the intensitymodulated and deflected electron beams secondary emissive currentsproportional to the amplitude of said oscillations and sinusoidalfunctions in quadrature of the second of said angles proportional tosaid varying and oscillating electrical quantity, combining the saidsecondary emissioncurrents to produce a resulting quantity comprising acomponent Varying as a sinusoidal function of the 5 rived from saidoscillations.

EDOUARD LABIN.

REFERENCES CITED The following references are of record in the *10 me pfthis patent:

UNITED STATES PATENTS Number Name Date 2,294,209 Roder Aug, 25, 1942 152,337,272 Roberts Dec, 21, 1943 2,257,795 Gray Oct. 7, 1941

